The present invention relates generally to digital signal processing apparatuses. More particularly, the invention relates to an apparatus in which, from among modulated digital signals obtained by digital signal processing from an analog information signal by a modulation method such as pulse code modulation (PCM) or difference modulation, a plurality of coded signals of every constant period are consolidated as one block and recorded in this block unit on a recording medium, and which, in the operation of reproducing these recorded signals, is capable of accurately reproducing without error the above mentioned analog information signal from the reproduced plurality of block signals even when a portion of the reproduced signals is missing.
In a known recording and reproducing apparatus for audio signals which records and reproduces an analog audio signal, as it is, on and from a traveling magnetic tape by means of a stationary head, there have unavoidably been a number of problems such as wow and flutter of tape travel and noise and distortion arising in the tape and head system. As a natural consequence, these problems impose limitations when it is desired to increase the quality of recording and reproducing audio signals.
Accordingly, a method which comprises converting an analog audio signal in a modulated digital signal (hereinafter referred to simply as "digital signal") by a modulation method such as pulse code modulation (PCM) and recording and reproducing this digital signal is being developed. By this method, problems such as those relating to the S/N ratio and distortion due to nonlinearity of the recording medium are solved to a considerable degree.
For recording and reproducing this digital signal, recording and reproducing systems of wider frequency band or greater number of channels in comparison with those of a systems for recording and reproducing analog signals are required. Accordingly, for this recording and reproducing apparatus, the use of a so-called video tape recorder (hereinafter referred to as VTR) which has heretofore been widely sold on the market as an apparatus for recording and/or reproducing composite video signals has reached a practical stage. The term "composite video signal" is used in the present specification to designate a signal which results from the addition of synchronizing signals such as a vertical synchronizing signal equalizing pulses, and a horizontal synchronizing signal to a video information signal. A VRT of this character is capable of recording/reproducing video signals of wide frequency band by means of rotary heads which trace with a relatively high relative tracing speed over oblique tracks on a traveling magnetic tape.
Recording of an audio signal by using this VTR is accomplished by converting the analog audio signal to be recorded into a digital signal, and causing this digital signal to exist between synchronizing signals which are the same as the synchronizing signals of an ordinary composite video signal in an adapter apparatus connected to the VTR, supplying the composite digital signal thus obtained to the VTR, and recording this signal by means of rotary heads on the magnetic tape. At the time of reproduction, the signal reproduced from the magnetic tape by rotary heads in the VTR is supplied to the adapter apparatus, where the synchronizing signals are removed, and the resulting digital signal is converted into an analog signal, thereby being restored into the original audio signal.
In the case where dust particles adhere to the surface of the above mentioned magnetic tape, or in the case where there are irregularities in the magnetic material of the tape, a signal loss or dropout occurs in one portion of the reproduced signal. In the case where this occurs, and the reproduction of the most significant bit (MSB), for example, is lost, the signal obtained by decoding will become a signal of considerably large erroneous value. If this signal is converted, as it is, into an analog signal and reproduced, it will accompany with a high noise voltage, and the reproduced sound will have an unpleasant quality.
As a countermeasure, there has heretofore been a method wherein the error is detected, and the modulated digital signal reproduced with error is substituted into that of the preceding sample. This method is called as a preceding value holding method. Another method employed heretofore substitutes the modulated digital signal reproduced with error into the center value of the values before and after sampling of the digital signal. This method is called as a center value correction method.
It has been the practice heretofore to resort to a method as described above to moderate the deterioration of the sound quality as sensed by ear of the reproduced sound at the time when partial dropout occurrs in the reproduced modulated digital signal. These methods known in the prior art, however, could not be ordinarily expected to be satisfactorily effective in lessening the deterioration of the reproduced sound of an audio signal whose level or frequency is abruptly varying.
Accordingly, there has heretofore been a method which comprises: forming a signal P for correction by a modulus 2 addition from m generation elements by a predetermined mode out of an aggregation $ comprising a plurality of signals (digital signals of one sampling period) coded every constant period out of the digital signals obtained coding after sampling the analog information signal, the m generation elements being coded signals constituting the aggregation $; similarly using once all of the constituent elements of the aggregation $ thereby to form a group of the signal P for correction; composing a block from one correction signal P from the group, an element other than its generation elements, and an error checking signal; recording by block units a plurality of blocks which have been similarly constituted; and, in the case where the generation elements of the aggregation $ are lost at the time of reproduction, using the correction signals P having these as generation elements to restore the same to the original signals. This type of the conventional apparatus is described, for example, in the specification of U.S. Pat. No. 3,697,948.
This conventional apparatus is not capable of restoring when the signal for correction concurrently includes more than two erroneous generation elements. Specifically, since a single block contains only one kind of correction signal, the conventional apparatus has been accompanied by difficulties in that the probability of failure in restoration is rather high.